Added upgrade step for Yii 2.0.17 about yii\web\DbSession (#17228) [skip ci]
Added upgrade step for Yii 2.0.17 about yii\web\DbSession (#17228) [skip ci]

https://github.com/yiisoft/yii2/pull/17188 might brake existing projects when the yii\web\DbSession class is extended.
arogachev/yii2,PowerGamer1/yii2,brandonkelly/yii2,LAV45/yii2,rob006/yii2-dev,Alex-Code/yii2,rob006/yii2-dev,vchenin/yii2,Alex-Code/yii2,wujuguang/yii2,NabiKAZ/yii2,yiijan/yii2,lubosdz/yii2,bizley/yii2,wujuguang/yii2,yiisoft/yii2,mikehaertl/yii2,yiisoft/yii2,dungphanxuan/yii2,fetus-hina/yii2,LAV45/yii2,yiisoft/yii2,Alex-Code/yii2,arogachev/yii2,lubosdz/yii2,yiijan/yii2,dungphanxuan/yii2,arogachev/yii2,rob006/yii2,lubosdz/yii2,fetus-hina/yii2,mikehaertl/yii2,LAV45/yii2,vchenin/yii2,lubosdz/yii2,vchenin/yii2,wujuguang/yii2,bizley/yii2,vchenin/yii2,NabiKAZ/yii2,yiijan/yii2,rhertogh/yii2,xiaoliwang/yii2,rob006/yii2-dev,LAV45/yii2,NabiKAZ/yii2,LAV45/yii2,yiijan/yii2,rob006/yii2-dev,Alex-Code/yii2,dungphanxuan/yii2,brandonkelly/yii2,wujuguang/yii2,mikehaertl/yii2,mikk150/yii2,NabiKAZ/yii2,yiijan/yii2,wujuguang/yii2,mikk150/yii2,yiisoft/yii2,NabiKAZ/yii2,rob006/yii2,rob006/yii2,bizley/yii2,PowerGamer1/yii2,Alex-Code/yii2,arogachev/yii2,dungphanxuan/yii2,mikk150/yii2,xiaoliwang/yii2,bizley/yii2,brandonkelly/yii2,mikehaertl/yii2,xiaoliwang/yii2,dungphanxuan/yii2,rhertogh/yii2,PowerGamer1/yii2,mikk150/yii2,mikk150/yii2,rhertogh/yii2,rob006/yii2,rhertogh/yii2,fetus-hina/yii2,fetus-hina/yii2,rob006/yii2-dev,PowerGamer1/yii2,xiaoliwang/yii2,brandonkelly/yii2